FIG. 1 of the accompanying drawings is a schematic circuit diagram of a conventional DC brushless motor control circuit for use in applications such as air moving fans requiring relatively low power, low voltage DC.
In the motor control circuit of FIG. 1, a supply voltage is fed to a top rail, for example between 18 to 32 volts DC or 36 to 60 volts DC depending on the motor requirements. The motor windings 1 are connected to a full bridge circuit consisting of four n-type FETs 2a,2b,2c,2d driven by a pair of charge pump circuits (IR2104) 3a,3b. The charge pump circuits each receive a direct feed from a DC motor controller IC 4 such as a INT 100 and are each connected to the top rail voltage supply by a respective capacitor 5a,5b. The outputs H1, H2, L1, L2 of the controller IC 4 are connected to the bridge.
A voltage regulator 6 comprising a linear DC-DC converter supplies the controller IC 4 with 12 to 15 volts DC. However, a voltage higher than the top rail must be provided to turn on the two FETs 2a, 2b in the top half of the full bridge, hence the need for the charge pump circuits for the FETs in the top half of the full bridge. The drains of the FETs in the bottom half of the bridge are connected to a resistor 7 to provide an output for sensing the current being drawn by the motor windings 1. The resistor 7 is connected to the current sense input of the IC4.
The n-type FETs can be replaced with p-type FETs but these are expensive, not readily available and less efficient than n-type FETs.
There are problems associated with the circuit of FIG. 1 and other conventional motor control circuits which utilise charge pump circuits for driving the FETs 2a,2b and a linear DC-DC regulator to provide a stable voltage to the control circuit. Charge pump circuits require bulky, expensive components and are not very efficient at transferring energy. In a motor control application the linear DC-DC regulator circuit has to generate a stable voltage over a wide input voltage range. Adherence to this requirement results in a design which is inefficient at high supply voltage levels. Effectively, the voltage regulator must be over-specified to drive the charge pump circuits, thereby causing the voltage regulator to generate too much energy which requires dissipation. It is always inconvenient to dissipate heat from the confines of an enclosure housing a motor and its associated control circuitry, let alone having to dissipate heat from an over-specified voltage regulator.